1. Field of the Invention
The present invention relates to a multilayer bandpass filter in which a plurality of dielectric layers and a plurality of electrode layers are laminated.
2. Description of the Related Art
Recently, high-frequency bandpass filters suitable for miniaturization and cost reduction have been manufactured by providing a plurality of LC parallel resonators in a stack of dielectric layers and electrode layers.
Japanese Unexamined Patent Application Publication No. 2007-13962 discloses such a multilayer bandpass filter.
The configuration of a multilayer bandpass filter 200 disclosed in Japanese Unexamined Patent Application Publication No. 2007-13962 will be described with reference to FIGS. 9 and 10.
FIG. 9 is an exploded perspective view of the multilayer bandpass filter 200. FIG. 10 is an equivalent circuit diagram of the multilayer bandpass filter 200. As illustrated in FIG. 9, the multilayer bandpass filter 200 includes a stack of a dummy layer 210, a dielectric layer 211 on which a floating ground electrode 220 is formed, a dielectric layer 212 on which two half-wavelength resonators 222 and 224 are formed, a dielectric layer 213 on which a capacitor electrode 228 is formed, a dielectric layer 214 on which capacitor electrodes 230 and 232 are formed, a dielectric layer 215 on which capacitor electrodes 234 and 236 are formed, and a dielectric layer 216 on which input/output electrodes 238 and 240 and a ground electrode 242 are formed.
The substantially reverse L-shaped half-wavelength resonator 222 and the substantially L-shaped half-wavelength resonator 224 are symmetrically disposed at the center of the dielectric layer 212 so that they are spaced apart from each other by a predetermined distance. As a result, the half-wavelength resonators 222 and 224 are coupled to each other by magnetic field coupling. End portions at the long sides of these resonators are coupled to each other by a conductor electrode 226, and are electrically connected to the ground electrode 242 through a via electrode 253. End portions at short sides of the half-wavelength resonators 222 and 224 are provided with via electrodes 251 and 255, respectively, and are electrically connected to the input/output load capacitor electrodes 234 and 236 on the dielectric layer 215 via the via electrodes 251 and 255, respectively.
The input/output load capacitor electrodes 234 and 236 are arranged on the dielectric layer 215 so that the input/output load capacitor electrodes 234 and 236 face the input/output capacitor electrodes 230 and 232 via the dielectric layer 214, respectively. The input capacitor electrode 230 and the input load capacitor electrode 234 face each other, so that an input capacitor C1 is formed. The output capacitor electrode 232 and the output load capacitor electrode 236 face each other, so that an output capacitor C2 is formed.
The input/output load capacitor electrode 234 faces the ground electrode 242 via the dielectric layer 215, so that an input/output load capacitor C4 is formed. The input/output load capacitor electrode 236 faces the ground electrode 242, so that an input/output load capacitor C5 is formed.
The input/output coupling capacitor electrode 228 is substantially rectangular, and is arranged on the dielectric layer 213 so that it faces the input/output capacitor electrodes 230 and 232 via the dielectric layer 213. The input/output capacitor electrodes 230 and 232 and the input/output coupling capacitor electrode 228 form an input/output coupling capacitor C3.
FIG. 10 is an equivalent circuit diagram of the multilayer bandpass filter illustrated in FIG. 9. In FIG. 10, an inductor L1 corresponds to a via electrode 261 for electrically connecting the input/output electrode 238 and the input/output capacitor electrode 230 illustrated in FIG. 9 to each other, and an inductor L2 corresponds to a via electrode 263 for electrically connecting the input/output electrode 240 and the input/output capacitor electrode 232 illustrated in FIG. 9 to each other. An inductor L3 corresponds to the via electrode 253 for electrically connecting the ground electrode 242 and the conductor electrode 226 for coupling the end portions at the long sides of the half-wavelength resonators 222 and 224 illustrated in FIG. 9. An inductor L4 corresponds to the via electrode 251 for connecting the end portion at the short side of the half-wavelength resonator 222 and the input/output load capacitor electrode 234 illustrated in FIG. 9. An inductor L5 corresponds to the via electrode 255 for connecting the end portion at the short side of the half-wavelength resonator 224 and the input/output load capacitor electrode 236 illustrated in FIG. 9.
In general high-frequency circuit systems, the input and output impedance values of a filter are set to desired values so as to prevent the loss of an electric signal caused by impedance mismatching and achieve impedance matching between electronic components included in the system. In the multilayer bandpass filter 200, input and output impedance values are set to desired values with the ratio between the values of the input capacitor C1 and the input/output load capacitor C4 illustrated in FIG. 10 and the ratio between the values of the output capacitor C2 and the input/output load capacitor C5 illustrated in FIG. 10.
The input and output impedance values of the multilayer bandpass filter 200 are also determined by the degree of electromagnetic coupling between the half-wavelength resonators 222 and 224, that is, the distance between the half-wavelength resonators 222 and 224.
Accordingly, in order to set desired impedance values, it is necessary to set a predetermined distance between the half-wavelength resonators. When the distance between resonators is long, the entire size of a component is increased.
When the distance between resonators is changed, the positions of the via electrodes 251 and 255, each of which is disposed at the short side of a corresponding one of the resonators, are changed. As a result, it is necessary to change the positions of the input/output load capacitor electrode 234 connected to the via electrode 251, the input/output load capacitor electrode 236 connected to the via electrode 255, and other electrodes on the paths of the via electrodes 251 and 255. Consequently, it is necessary to redesign a component. This leads to reduction in design flexibility.